This invention is generally directed to liquid developer compositions and, more specifically, the present invention relates to processes for the preparation of a liquid developer containing, for example, NORPAR.RTM. with supercritical fluid extraction. In embodiments, the present invention relates to processes for the preparation of liquid developers with high solids contents of, for example, about equal to or greater than 50 weight percent. The process of the present invention in embodiments comprises the extraction from the developer of the ink vehicle like NORPAR.RTM. by relatively mild supercritical extraction, and more specifically supercritical carbon dioxide solvent extraction to thereby concentrate the liquid ink solids of resin, pigment, and charge adjuvant or charge additive. The process of the present invention can be selected to fabricate liquid ink concentrates that can then be selected for electrographic, especially xerographic, systems wherein liquid hydrocarbons are recycled. Advantages of using high solids ink replenishment processes include avoiding the build up of excess liquid carrier by replenishing the developer housing with a percent solid ink that is equal to the percentage solid concentration of the image fixed to the paper, typically about 50 percent, and greater stability with respect to storage of the high solids ink versus a dilute suspension. The inks obtained with the process of the the present invention can be selected for a number of known imaging systems, such as xerographic imaging and printing processes, wherein latent images are rendered visible and which images possess excellent characteristics of image quality, solid area coverage and resolution.
Advantages, or problems solved with the processes of the present invention include the capability to consistently and uniformly achieve a liquid ink with a 50 or greater solids content without the potential for contamination by organic solvents used for extraction as, for example, is the situation with the processes as illustrated in U.S. Pat. No. 5,206,108 wherein hexane is used in combination with filtration or centrifugation to remove the aliphatic hydrocarbon vehicle in which the ink was generated. Alternative known means of liquid developer concentration include the use of a centrifuge, however, this operation is limited by the viscosity of the ink vehicle, the small particle size, and the similar densities of liquid and particle to typically less than about 30 percent solids. Various types of filter presses can achieve higher average solid concentrations, however, the inevitable nonuniformity of pressure applied throughout the filter cake results in a gradient of solids concentrations. Additionally, the compaction produced by a filter press may render the high solids ink more difficult to redisperse in the hydrocarbon vehicle for development in a printer.
Processes for the preparation of liquid inks by, for example, pressure filtration, or centrifugation are known. For example, U.S. Pat. No. 5,206,108 illustrates the use of hexane in combination with filtration or centrifugation to remove the aliphatic hydrocarbon vehicle in which the ink was generated, which process results in the contamination of the ink vehicle by organic solvents used for extraction. These contaminants possess a significant vapor pressure and are a health hazard that precludes their use in most environments. Alternative means of concentration include the use of a centrifuge, however, this operation is limited by the viscosity of the ink vehicle, the small ink particle size, and the similar densities of liquid and ink particle.
Various types of filter presses can achieve higher average solid concentrations, however, the nonuniformity of pressure applied throughout the filter cake results in a gradient of solids concentrations. Additionally, the compaction produced by a filter press may render the high solids ink more difficult to redisperse in the hydrocarbon vehicle for development in a copier or a printer.
Processes for the preparation of certain liquid electrostatic developers include mixing in a nonpolar liquid the thermoplastic resin, charging additive, and optional colorant and adjuvant in a manner that the resulting mixture contains, for example, from about 15 to about 30 percent by weight of solids; heating the mixture to a temperature of from about 70.degree. C. to about 130.degree. C. until a uniform dispersion is formed; adding an additional amount of nonpolar liquid sufficient to decrease the total solids concentration of the developer to about 10 to about 20 percent by weight; cooling the dispersion to about 10.degree. C. to about 50.degree. C.; adding the charge director compound to the dispersion; and diluting the dispersion.
In the initial mixture, the resin, colorant and charge adjuvant may be added separately to an appropriate vessel such as, for example, an attritor, heated ball mill, heated vibratory mill, such as a Sweco Mill manufactured by Sweco Company, Los Angeles, Calif., equipped with particulate media for dispersing and grinding, a Ross double planetary mixer manufactured by Charles Ross and Son, Hauppauge, N.Y., or a two roll heated mill, which usually requires no particulate media. Useful particulate media include materials like a spherical cylinder of stainless steel, carbon steel, alumina, ceramic, zirconia, silica and sillimanite. Carbon steel particulate media are particularly useful when colorants other than black are used. A typical diameter range for the particulate media is in the range of 0.04 to 0.5 inch (approximately 1.0 to approximately 13 millimeters). Other processes include extrusion, for example, U.S. Pat. No. 5,017,451 illustrates extrusion for melt mixing and a Microfluidizer.RTM. for particle size reduction. Also, in U.S. Pat. No. 5,387,489, the disclosure of which is totally incorporated herein by reference, there can be selected a piston homogenizer for particle size reduction.
When using an attritor, sufficient nonpolar liquid is added to provide a dispersion of from about 10 to about 50 percent solids. This mixture is then subjected to elevated temperatures during the initial mixing procedure to plasticize and soften the resin. The mixture is sufficiently heated to provide a uniform dispersion of all the solid materials of, for example, colorant, charge director, adjuvant and resin. However, the temperature at which this step is effected should not be high as to degrade the nonpolar liquid or decompose the resin or colorant if present. Accordingly, the mixture in embodiments is heated to a temperature of from about 70.degree. C. to about 130.degree. C., and preferably from about 75.degree. C. to about 110.degree. C. The mixture may be ground in a heated ball mill or heated attritor at this temperature for about 15 minutes to 5 hours, and preferably about 60 to about 180 minutes. After grinding at the above temperatures, an additional amount of nonpolar liquid may be added to the dispersion. The amount of nonpolar liquid to be added should be sufficient in embodiments to decrease the total solids concentration of the dispersion to about 10 to about 20 percent by weight.
The dispersion is then cooled to about 10.degree. C. to about 50.degree. C., and preferably to about 15.degree. C. to about 30.degree. C., while mixing is continued until the resin admixture solidifies or hardens. Upon cooling, the resin admixture precipitates out of the dispersant liquid. Cooling is accomplished by methods such as the use of a cooling fluid like water, glycols, such as ethylene gylcol, in a jacket surrounding the mixing vessel. Cooling is accomplished, for example, in the same vessel, such as an attritor, while simultaneously grinding with particulate media to prevent the formation of a gel or solid mass; without stirring to form a gel or solid mass, followed by shredding the gel or solid mass and grinding by means of particulate media; or with stirring to form a viscous mixture and grinding by means of particulate media. The resin precipitate is cold ground for about 1 to 36 hours, and preferably from about 2 to about 6 hours. Additional liquid may be added at any time during the preparation of the liquid developer to facilitate grinding or to dilute the developer to the appropriate percent solids needed for developing. Other processes of preparation are generally illustrated in U.S. Pat. Nos. 4,760,009; 5,017,451; 4,923,778; 4,783,389, the disclosures of which are totally incorporated herein by reference.
A latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. These dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive imaging member or layer with a uniform electrostatic charge, and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet. Also known are ionographic imaging systems.
Typical liquid developers can comprise a thermoplastic resin and a dispersant nonpolar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present in the developer. The colored toner particles, or solids are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 30 .mu.m (microns) average by area size as measured with the Malvern 3600E particle sizer.
The following patents relating to liquid developers and processes thereof are mentioned.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner pigmented particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers can be prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture; and then cooling the mixture whereby the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
Moreover, in U.S. Pat. No. 4,707,429 there are illustrated, for example, liquid developers with an aluminum stearate charge adjuvant. Liquid developers with charge directors are also illustrated in U.S. Pat. No. 5,045,425.
In copending patent application U.S. Ser. No. 986,316, now U.S. Pat. No. 5,998,081, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., the developer having a melting point of at least about 25.degree. C., wherein contacting occurs while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.
The disclosures of each of the copending patent applications and patents are totally incorporated herein by reference.